In power electronic devices, such as converters (inverters), energy must frequently be stored in intermediate circuit capacitors. This process is used for buffering and thereby filtering currents and disturbances caused by these currents through switching processes of a power electronic component, such as an inverter for driving an electric motor.
In the prior art, electrolytic capacitors are frequently used in assembling such intermediate circuit capacitors. Electrolytic capacitors have the advantage of high energy density, a wide operating temperature range and lower costs. Electrolytic capacitors are usually available in symmetrical cylindrical configurations and have either one terminal (+/−) on each end face (axial), or two terminals (+/−) on one end face (radial).
Due to the physical design of electrolytic capacitors, they possess parasitic resistance. Parasitic resistance can be described electrotechnically in terms of “ESR” (equivalent series resistance). The ESR of commercially available electrolytic capacitors is usually around a few ohms.
Switching processes in power electronic components result in a current loading of the intermediate circuit capacitors and thus an application of current against the ESR. This results in a dissipation power loss in the electrolytic capacitors, causing them to heat. The heating, in particular the heating of the electrolyte, leads to a deterioration of electrical properties, in particular the current-carrying capacity of the electrolytic capacitors. As a result, the filtering, interception and buffering of switching processes is deteriorated. This can lead to corresponding disturbances and interference in the power electronic component itself or in other components. The heating of the electrolytic capacitors or the electrolyte also leads to a shortening of the lifespan of the electrolytic capacitor. The risk is that the electrolyte may dry out or the dielectric strength of the electrolyte may be reduced. This leads to power electronic components needing to be serviced or replaced earlier and more frequently.
When inverters and thus intermediate circuit capacitors are used in conjunction with electric refrigeration compressors, it is therefore advisable to use the cooling power of the refrigeration compressors to actively cool these intermediate circuit capacitors, thereby diminishing the above-described negative effects of overheating.
One disadvantage of the cited prior art is that electrolytic capacitors are poorly and/or unevenly cooled. The uneven cooling may occur in each capacitor individually, that is to say, heat may be dissipated better in some parts of a capacitor than in other parts of the capacitor. Temperature differences may also exist among individual capacitors in an intermediate circuit filter.
A further known disadvantage of the prior art is that electrolytic capacitors that are mounted on an outer surface using retaining clamps are inadequately secured against vibrations.
For active cooling, the surface of the electrolytic capacitor must be attached with the lowest possible thermal resistance to the cooling surface or the heat sink. This can be achieved with either flexible thermopads or special thermally conductive adhesive. These cooling pads or thermal adhesives typically have thermal conductivities of 1 to 10 W/mK, and thus a lower thermal conductivity than a heat sink or the housing of the refrigeration compressor. Such housings/heat sinks are typically made of copper having a thermal conductivity of 400 W/mK or aluminum having a thermal conductivity of typically 235 W/mK.
It is further known to mount electrolytic capacitors with the lateral surface thereof lying flat or adhesively bonded in a sink. In the latter case, the adhesive is first applied using a metering unit, after which the capacitor is placed in the adhesive. It is further known to adhesively bond electronic components by encapsulation or potting.
A further disadvantage of the aforementioned prior art is that adhesive bonding on only a small part of the lateral surface of the capacitor results in uneven thermal attachment of the component. Moreover, the potting or encapsulation of electrical components requires that they be placed in relatively large cavities as compared with the size of the component. This results in a relatively long thermal pathway through the overmolded or potted material to the actual heat sink. In addition, with the overmolding/potting of multiple components, pathways of different lengths through the thermal material results in the disadvantage of thermally different heat dissipation in the components. Furthermore, the consumption of potting compound/overmolding compound is relatively high, which leads to increased costs.